This invention relates to electric switches, and particularly to electric switches of the type employing a bridging contact rectilinearly movable into and out of engagement with a pair of fixed contacts.
In electrical power distribution systems, the limiting of peak demand is commonly controlled by selectively disconnecting non-critical residential and office branch circuits. The typical demand control system uses a switching device such as an electromagnetically operated electric switch between the branch circuit and the electric supply system. The switch is normally closed so that the branch circuit is normally connected to the electric supply system and is opened only during high demand periods. Although the branch circuits controlled by the switching devices are equipped with circuit breakers for protection against current overloads, the switching devices may be required to handle high levels of fault currents on the order of 12,000 amperes for short time periods such as five line cycles. When a switch having a bridging contact held against a pair of fixed contacts by a spring is subjected to a fault current, the contact pressure provided by the spring can be overcome by the electrodynamic forces of repulsion developed between the bridging contact and the fixed contacts. Contact separation under these conditions can result in extensive damage to the contacts, if not failure of the switch. While a stronger biasing spring could be employed to ensure adequate contact pressure under fault current conditions, a large mechanical force typically supplied by an electromagnet is required to operate the switch resulting in a larger, more costly switching device.
It has also been proposed to enhance the fault current capability of an electric switch by arranging its current carrying elements to develop electrodynamic forces opposing the forces of repulsion developed between the bridging contact and the fixed contacts. The Ainsworth U.S. Pat. No. 1,762,604 issued June 10, 1930 discloses an auxiliary bridging contact apparatus carried by a main bridging contact member and comprising two movable contact bars pivotally supported in the respective loop-shaped ends of a conductive support. The size and cost of such a complex arrangement is not suited to electric switches intended for use in residential and light industrial applications such as the aforementioned demand control systems. In the Bremer U.S. Pat. No. 3,419,828 issued Dec. 31, 1968, there is disclosed an electric switch with a rigid conducting strip attached to one fixed contact and bent to overlie the bridging contact in cantilever fashion for developing a repulsive force which acts to maintain the bridging contact in engagement with the fixed contacts under heavy excess current conditions. The magnitude of the repulsive force exerted upon the bridging contact in the switch of the latter patent is limited since the bridging contact in its closed position must be spaced from the rigid conducting strip a distance greater than the travel of the bridging contact between its open and closed positions. The location of the switch terminals adjacent each other rather than on opposite sides of the switch is also disadvantageous for some applications.